In recent years, as a battery anticipated to have a small size, a light weight, and high capacity, a non-aqueous electrolytic solution-based secondary battery such as a lithium ion battery has been proposed and put into practical use. The lithium ion battery is configured to have a positive electrode and a negative electrode which have properties capable of reversibly intercalating and deintercalating lithium ions, and a non-aqueous electrolyte.
As a negative electrode active material for negative electrode materials of the lithium ion battery, a carbon-based material, lithium titanate (Li4Ti5O12), or the like is used.
On the other hand, as a positive electrode active material for positive electrode materials of the lithium ion battery, a lithium-containing metal oxide such as lithium cobaltate (LiCoO2), a lithium oxoacid salt-based compound such as lithium iron phosphate (LiFePO4), or the like is used.
Compared with secondary batteries of the related art such as lead batteries, nickel-cadmium batteries, and nickel-hydrogen batteries, the lithium ion batteries have a lighter weight, a smaller size, and higher energy. Therefore, the lithium ion batteries are used not only as small-size power supplies used in portable electronic devices such as mobile phones and notebook personal computers but also as large-size stationary emergency power supplies.
In addition, recently, studies have been underway regarding the use of lithium ion batteries as high-output power supplies for plug-in hybrid vehicles, hybrid vehicles, industrial device, electric power tools, and the like. Since batteries used as the above-described high-output power supplies are frequently used in applications that are expected to be used outdoors, the batteries are required to have high-speed charge and discharge characteristics not only at room temperature but also at a low temperature in a case in which the batteries are expected to be used in cold areas.
Among the electrode active materials, the lithium oxoacid salt-based compound (particularly, lithium iron phosphate) is attracting attention due to the excellent safety and the absence of problems with its resource and cost. However, the lithium oxoacid salt-based compound has a problem with low electron conductivity due to its crystal structure (olivine-type crystal structure).
Therefore, in order to increase the electron conductivity of an electrode material for which the lithium oxoacid salt-based compound is used as an electrode active material, means of Patent Document 1 has been proposed. In Patent Document 1, particle surfaces of an electrode active material made of lithium iron phosphate are covered with an organic compound that is a carbon source, then, the organic compound is carbonized so as to form a carbonaceous film on the surface of the electrode active material, and carbon in the carbonaceous film is made to act as an electron conductive substance.